Nanotechnology Spotlight

Innovative substrate engineering for high quality 2D nanomaterials

(Nanowerk Spotlight) One of the greatest challenges in commercializing graphene is how to produce high quality material, on an industrial scale, at low cost, and in a reproducible manner. The quality of graphene plays a crucial role as the presence of defects, impurities, domain boundaries, multiple domains, structural disorders, or wrinkles in the graphene sheet can have undesired or unexpected effects on its electronic and optical properties (read more: "Mass production of high quality graphene: An analysis of worldwide patents").

Chemical vapour deposition (CVD) has been identified as one of the most promising techniques for industrial nanomaterial synthesis. The procedure is very simple in principle and only requires gaseous precursor mixtures to be contacted with a hot catalytic substrate that facilitates the precursor decomposition leaving a single layer of a two-dimensional (2D) nanomaterial.

For example, methane gas – the natural gas that is typically used in households for cooking or heating – can be used to deposit a crystal of carbon atoms on copper (i.e. graphene). Current synthesis methods suffer from a strong dependence on the quality of graphene on the underlying metallic substrate and very slow growth rates (i.e. a few micrometers per minute).

Researchers from the Nanomaterials by Design group at Oxford University, led by professor Nicole Grobert, have now demonstrated a novel, low-cost substrate processing procedure to achieve rapid, efficient synthesis of millimeter-sized single crystal graphene.

"We found a way to engineer the substrate in a way that results in a significant improvement in the morphology of graphene flakes," Vitaliy Babenko, a PhD student in Grobert's group, tells Nanowerk. "Furthermore, the growth rate increases by more than an order of magnitude – about 120 µm per minute – compared to the typically reported techniques."

Babenko explains that the improvement is achieved by using a substrate that is prepared by depositing a thin film of silica, a compound that principally makes up sand, onto a platinum foil. When the stack is heated in an ordinary CVD system a reaction occurs between the film and the foil, leading to the formation of platinum silicide, which is an example of a eutectic compound. The latter means that platinum silicide melts at lower temperature than either platinum or silica.

As a result, a thin liquid layer is formed on the foil that screens the platinum lattice and fills topographic defects allowing large graphene crystals to form in situ.

Scanning electron micrographs of graphene without (left) and with (right) silicidation on platinum. Common solid metallic substrates consist of grains and their boundaries that look like tiles of varying contrast in the background and present a non-uniform surface for graphene to grow. This causes defects in graphene due to misalignment with the lattice of the grains and when graphene domains cross from one grain to another (image on the left). Molten silicide forms a mobile surface for graphene growth that dramatically improves the uniformity, size and growth rate of monolayer graphene crystals (image on the right). (Image: University of Oxford, Nanomaterials by Design Group)

"An additional benefit of this system is that the surface seems to become 'stickier', increasing the dwell time of carbon atoms on the surface, thus dramatically improving the growth rate and allowing savings in costs for consumable materials, energy and time," Babenko points out.

For comparison, on copper the synthesis of mm-sized graphene flakes may take up to 12 hours while on silicidated platinum it only takes 15 minutes as described in the paper.

"To our knowledge no current reports focusing on graphene synthesis via CVD consider the effects of the formation of the silicides, though commonly used reaction tubes are made of fused silica," Grobert notes. "It is likely that previous results were influenced by these silicides but were overlooked by other reports. In particular, we explicitly verify the dependence of the amount of silica on the crystallinity of graphene flakes on platinum. All of these beneficial effects may make the silicidation feasible commercially, especially if the procedure is extended to other metals."

"Most metals on the periodic table form silicides which can lead to many other useful systems to be discovered," says Babenko. "Additionally there are many other non-silicide, binary, ternary or higher eutectic systems that can potentially be utilized in similar way. This generic technique paves the way for novel types of substrate engineering of eutectic compounds to produce large-area 2D-nanomaterials for applications where the highest quality is needed."

A patent application related to this work was filed in 2014 and is part of a growing patent portfolio of nanomaterials and their production technologies from Professor Nicole Grobertís Nanomaterials by Design Group. Isis Innovation, the technology commercialisation company of the University of Oxford, and Industrial partners form an essential part for the development the Groupís products for potential applications. Professor Grobert also plans to manufacture and sell her range of specialty nanomaterials as part of a new business venture.